Crosslinkable polyolefin composition comprising silane groups forming an acid or a base upon hydrolysation

ABSTRACT

The present invention relates to a polyolefin composition comprising (A) a crosslinkable polyolefin with hydrolysable silane groups which upon hydrolysation form an acid or a base, characterised in that the acid or base generates a gel content of at least 40% after 40 hours in the 90° C. cross-linking test, wherein the acid or base is added in an amount of 4.5 mmol/kg to an ethylene/vinyltrimethoxysilane copolymer with a MFR 2 =2 g/10 min, a density of 923 g/cm 3 , and 2 wt. % of vinyltrimethoxysilane and then is cross-linked in a waterbath at 90° C., and to a polyolefin composition comprising (i) a crosslinkable polyolefin with hydrolysable silane groups, and (ii) a non-polymeric compound with hydrolysable silane groups which upon hydrolysation form an acid or a base, characterised in that the acid or base generates a gel content of at least 40% after 40 hours in the 90° C. cross-linking test, wherein the acid or base is added in an amount of 4.5 mmol/kg to an ethylene/vinyltrimethoxysilane copolymer with a MFR 2 =2 g/10 min, a density of 923 g/cm 3 , and 2 wt. % of vinyltrimethoxysilane and then is cross-linked in a waterbath at 90° C.

The present invention relates to polyolefin compositions comprising acrosslinkable polyolefin with hydrolysable silane groups, to a wire orcable comprising such compositions, to the use of such compositions forthe production of a wire or cable, to the use of a hydrolysed silanegroup containing polyolefin or non-polymeric compound as silanolcondensation catalyst, and to the use of a non-polymeric silane groupcontaining compound as a scorch retarding agent.

It is known to cross-link polyolefins by means of additives as thisimproves several of the properties of the polyolefin, such as mechanicalstrength and chemical heat resistance. Cross-linking may be performed bycondensation of silanol groups contained in the polyolefin which can beobtained by hydrolysation of silane groups. A silane compound may beintroduced as a cross-linkable group into a polyolefin e.g. by graftingthe silane compound onto the polyolefin, or by copolymerisation ofolefin monomers and silane group containing monomers. Such techniquesare known e.g. from U.S. Pat. Nos. 4,413,066, 4,297,310, 4,351,876,4,397,981, 4,446,283 and 4,456,704.

In the present invention, the polyolefin composition is particularlyused for the production of a wire or cable, in particular a low, mediumor high voltage cable. Electric power cables for low voltages, i.e.voltages of below 3 kV, usually comprise an electric conductor which iscoated with an insulation layer. Such low voltage cables are alsodenoted as single wire cables. Optionally, two or more of such singlewire cables are surrounded by a common outermost sheath layer, thejacket.

A typical medium voltage power cable, usually used for voltages from 3to 36 kV, and a typical high voltage cable used for voltages higher than36 kV, comprises one or more conductors in a cable core that issurrounded by several layers of polymeric materials, including an innersemiconducting layer, followed by an insulating layer, and then an outersemiconducting layer. These layers are normally crosslinked. To theselayers, further layers may be added, such as a metallic tape or wireshield, and, finally, an outermost jacketing layer. The layers of thecable are based on different types of polymer compositions. Asinsulating materials, mainly cross-linked polyolefins such ascrosslinked low density polyethylene are used.

As hydrolysable silane groups in cross-linkable polyolefins todaypredominantly alkoxy groups are used. For cross-linking of polyolefinscontaining such hydrolysable silane groups, a silanol condensationcatalyst must be used. Conventional catalysts are, for example, tin-,zinc-, iron-, lead- or cobalt-organic compounds such as dibutyl tindilaurate (DBTDL). The cross-linking process may advantageously becarried out in the presence of Brönsted acid silanol condensationcatalysts. In contrast to the conventional metal-organic catalysts thesecatalysts allow cross-linking to quickly take place already at roomtemperature. Such acidic silanol condensation catalysts are disclosedfor example in WO 95/17463.

However, it is known that these Brönsted acids are comparatively strongacids, which gives rise to problems with corrosion and interaction withcolour masterbatches and other additives.

Furthermore, it is a known that in order to obtain a homogeneousend-product a good mixing of the components of the polyolefincomposition must be achieved, which is usually done by compounding thepolyolefin composition in an extruder. However, due to the presence ofcross-linkable silane groups in the composition and the elevatedtemperature and pressure in the extruder undesired cross-linking of thecomposition already in the extruder may take place, which is known as“scorch”.

It is hence an object of the present invention to provide a polyolefincomposition comprising a cross-linkable polyolefin with hydrolysablesilane groups which can be cross-linked using milder conditions, forexample by avoiding the addition of strong acids to the composition, butstill allowing for quick cross-linking desirably at room temperature,and for a high cross-linking degree.

It is a further object of the invention to provide a compound for apolyolefin composition comprising a cross-linkable polyolefin withhydrolysable silane groups which reduces undesired cross-linking, i.e.scorch, during extrusion of the composition.

It has now surprisingly been found that the above objects can beachieved by the use of a polyolefin composition in which a component ispresent containing hydrolysable silane groups which upon hydrolysationform an acid or a base.

The present invention therefore in a first embodiment provides apolyolefin composition comprising

-   -   (A) a crosslinkable polyolefin with hydrolysable silane groups        which upon hydrolysation form an acid or a base,        characterised in that the acid or base generates a gel content        of at least 40% after 40 hours in the 90° C. cross-linking test,        wherein the acid or base is added in an amount of 4.5 mmol/kg to        an ethylene/vinyltrimethoxysilane copolymer with a MFR₂=2 g/10        min, a density of 923 g/cm³, and 2 wt. % of        vinyltrimethoxysilane and then is cross-linked in a waterbath at        90° C.

Furthermore, the present invention provides in a second embodiment apolyolefin composition comprising

-   -   (i) a crosslinkable polyolefin with hydrolysable silane groups,        and    -   (ii) a non-polymeric compound with hydrolysable silane groups        which upon hydrolysation form an acid or a base,        characterised in that the acid or base generates a gel content        of at least 40% after 40 hours in the 90° C. cross-linking test,        wherein the acid or base is added in an amount of 4.5 mmol/kg to        an ethylene/vinyltrimethoxysilane copolymer with a MFR₂=2 g/10        min, a density of 923 g/cm³, and 2 wt. % of        vinyltrimethoxysilane and then is cross-linked in a waterbath at        90° C.

The silane groups which are present in the composition of the firstembodiment and which form an acid or a base upon hydrolysation undergohydrolysis faster than the commonly used alkoxy silane groups. Thus, afaster and easier cross-linking is obtained which allows for milderconditions during the cross-linking step, e.g. by use of a weaker acidor a base as a silanol condensation catalyst.

Furthermore, the acid or base formed upon hydrolysis in the compositionsof both embodiments may itself serve as a silanol condensation catalyst,so that the addition of a separately added silanol condensationcatalytic agent can be minimised or even dispensed with. Due to the factthat the catalyst is only “released” upon hydrolysis, a premature,unwanted cross-linking can be avoided or reduced.

In the composition of the second embodiment, the non-polymeric compoundwith hydrolysable silane groups which upon hydrolysation form an acid ora base due to the faster hydrolysation of such silane groups serves asan effective scorch retardant agent.

It is to be noted that in the 90° C. cross-linking test which isdescribed in still more detail in the Examples section below, the “free”acid or base is tested, i.e. not the silane compound. The test may alsoserve to identify suitable substituents for the silane compounds whichupon hydrolysis form an acid or base.

In the following, preferred embodiments of both the first and the secondembodiment of the composition of the invention are described unlessmentioned otherwise.

Preferably, the acid or the base formed in the composition uponhydrolysation of the silane group containing compound is a Brönsted acidor base.

If an acid is formed in the composition upon hydrolysation of the silanegroup containing compound this acid preferably has a pK_(a) value of 5or less, more preferably has a pK_(a) value of 4 or less, and mostpreferably has a pK_(a) value of 3 or less.

If a base is formed in the composition upon hydrolysation of the silanegroup containing compound this base preferably has a pK_(b) value of 10or less, more preferably has a pK_(b) value of 5 or less preferably hasa pK_(b) value of 4 or less.

The acid or base formed upon hydrolysation preferably is selected fromthe group of carboxylic acids, sulphonic acids, phosphorous acids,halogen acids, oxoacids, oximes, imines and amines, and more preferablyupon hydrolysation a carboxylic acid is formed.

The cross-linkable polyolefin of the compositions of the inventionpreferably comprises, still more preferably consists of, a polyethylenecontaining hydrolysable silane groups.

Preferably, the acid or base generates a gel content of at most 20%,more preferably of at most 10% after 1.5 hours in the 90° C.cross-linking test.

Furthermore, preferably, the acid or base generates a gel content of atmost 60% more preferably of at most 50% after 5 hours in the 90° C.cross-linking test.

In the first embodiment of the composition of the invention, to form thecrosslinkable polyolefin with hydrolysable silane groups which uponhydrolysation form an acid or a base, the hydrolysable silane groups maybe introduced into the polyolefin by copolymerisation of e.g. ethylenemonomers with silane group containing comonomers or by grafting, i.e. bychemical modification of the polymer by addition of silane groups mostlyin a radical reaction. Both techniques are well known in the art.

Preferably, the silane group containing polyolefin has been obtained bycopolymerisation.

Preferably, an unsaturated silane compound is used for the preparationof the crosslinkable polyolefin with hydrolysable silane groups whichupon hydrolysation form an acid or a base, which is represented by theformulaR¹SiR²R³R⁴  (I)wherein

R¹ is a non-hydrolysable hydrocarbyl group which comprises 2 to 30carbon atoms, which further comprises at least one unsaturated group,and which may contain heteroatoms so that functional groups such asester, amide, imide, ether, thioether or amine may be present in R¹,

R², R³ and R⁴ are organic residues which may be the same or differentfrom each other with the proviso that at least one thereof will generatean acid or a base upon hydrolysation.

In the preferred embodiment where the acid or base formed uponhydrolysation is a carboxylic acid, at least one of R², R³ and R⁴ is anacyloxy group. More preferably one or two of R², R³ and R⁴ are acyloxygroups, and the remainder of R², R³ and R⁴ are other groups.

The acyloxy group(s) are preferably selected from the group consistingof C1-50 alkanoyloxy, C3-50 alkenoyloxy, C3-50 alkynoyloxy and C7-60arenoyloxy groups, more preferably from the group consisting of C1-30alkanoyloxy, C3-30 alkenoyloxy, C3-30 alkynoyloxy and C7-40 arenoyloxygroups.

Still more preferably, the acyloxy group(s) are selected from the groupconsisting of C10-26 alkanoyloxy or alkenoyloxy groups, more preferablyC10-26 alkanoyloxy groups.

A preferred example for the unsaturated silane compounds for thepreparation of the crosslinkable polyolefin with hydrolysable silanegroups which upon hydrolysation forms a carboxylic acid is vinyldimethyl octadecanoyloxy silane.

In the embodiment where the acid or base formed upon hydrolysation is abase, preferably at least one of R², R³ and R⁴ is an amino group, sothat an amine is formed. More preferably one or two of R², R³ and R⁴ areamino groups, and the remainder of R², R³ and R⁴ are other groups.

The substituents at the N atom of the amino groups preferably areselected from the group consisting of C1-C12 alkyl, C2-C12 alkenyl,C2-C12 alkynyl and C7 to C18 arenyl groups, more preferably from thegroup consisting of C1-C6 alkyl, C2-C6 alkenyl, C2-C6 alkynyl and C7 toC12 arenyl groups.

Preferred examples for the unsaturated silane compounds for thepreparation of the crosslinkable polyolefin with hydrolysable silanegroups which upon hydrolysation forms an amine are allyl dimethyl(diisopropyl amino)silane and bis(dimethyl amino)vinyl methyl silane.

In the crosslinkable polyolefin with hydrolysable silane groups whichupon hydrolysation form an acid or a base, several types of hydrolysablesilane groups may be present including such hydrolysable silane groups,which upon hydrolysation do not form an acid or a base.

It is, however, preferred that at least 0.1 wt. %, more preferably atleast 0.5 wt. %, and most preferably at least 1 wt. % of the silanegroups present in the crosslinkable polyolefin with hydrolysable silanegroups which upon hydrolysation form an acid or a base, are such silanegroups which upon hydrolysation form an acid or a base.

Furthermore, it is preferred that at most 20 wt. %, more preferably atmost 15 wt. %, and most preferably at most 10 wt. % of the silane groupspresent in the crosslinkable polyolefin with hydrolysable silane groupswhich upon hydrolysation form an acid or a base, are such silane groupswhich upon hydrolysation form an acid or a base.

If further hydrolysable silane compounds are used for the preparation ofthe crosslinkable polyolefin with hydrolysable silane groups which uponhydrolysation form an acid or base, or for the preparation of otherpolyolefins with hydrolysable silane groups, these are represented bythe formulaCH₂═CHSi(OA)₃  (II)wherein A is a hydrocarbyl group having 1-8 carbon atoms, preferably 1-4carbon atoms.

The most preferred compounds are vinyl trimethoxy silane, vinylbismethoxyethoxy silane, vinyl triethoxy silane,gamma-(meth)acryl-oxypropyl trimethoxy silane, andgamma(meth)acryloxypropyl triethoxy silane, or combinations of two ormore thereof.

The copolymerisation of the olefin, e.g. ethylene, and the unsaturatedsilane compound(s) may be carried out under any suitable conditionsresulting in the copolymerisation of the two monomers.

The crosslinkable polyolefin with hydrolysable silane groups which uponhydrolysation form an acid or a base preferably contains 0.001 to 15 wt.% of silane group containing monomers, more preferably 0.01 to 10 wt. %,and most preferably 0.1 to 5 wt. %, and most preferably 0.1 to 3 wt. %.

The total amount of crosslinkable polyolefin with hydrolysable silanegroups which upon hydrolysation form an acid or a base in thecomposition is at least 25 wt. %, more preferably at least 50 wt. %.

The composition according to the second embodiment of the inventioncomprises a non-polymeric compound with hydrolysable silane groups whichupon hydrolysation form an acid or a base.

In the present invention, the term “non-polymeric compound” denotescompounds with 50 repeating units or less.

Preferably, the non-polymeric compound with hydrolysable silane groupswhich upon hydrolysation form an acid or a base, is represented by theformulaR¹SiR²R³R⁴  (III)wherein

R¹, R², R³ and R⁴ are organic residues which may be the same ordifferent from each other with the proviso that at least one thereofwill generate an acid or a base upon hydrolysation.

Preferably, R¹ is a non-hydrolysable hydrocarbyl group with or withoutsaturation which comprises 2 to 30 carbon atoms, and which may containheteroatoms so that functional groups such as ester, amide, imide,ether, thioether or amine may be present in R¹.

In the preferred embodiment where the acid or base formed uponhydrolysation is a carboxylic acid, at least one of R¹, R², R³ and R⁴ isan acyloxy group. More preferably one, two or three of R¹, R², R³ and R⁴are acyloxy groups, and the remainder of R¹, R², R³ and R⁴ are othergroups. The acyloxy group(s) are preferably selected from the groupconsisting of C1-50 alkanoyloxy, C3-50 alkenoyloxy, C3-50 alkynoyloxyand C7-60 arenoyloxy groups, more preferably from the group consistingof C1-30 alkanoyloxy, C3-30 alkenoyloxy, C3-30 alkynoyloxy and C7-40arenoyloxy groups.

Still more preferably, the acyloxy group(s) are selected from the groupconsisting of C10-26 alkanoyloxy or alkenoyloxy groups, more preferablyC10-26 alkanoyloxy groups.

Preferred examples for the non-polymeric compound with hydrolysablesilane groups which upon hydrolysation form an acid are octadecanoyloxytrimethyl silane, acetoxymethyl dimethyl acetoxy silane, bis(trichlorosilyl ethyl)benzene, t-butyl diphenyl chloro silane, dibenzyloxydiacetoxy silane, dibutoxy aluminoxy triethoxy silane, dibutyl silylbis(trifluoromethanesulfonate), octadecyl dimethyl chloro silane,trimethyl silyl benzenesulfonate, tris(trimethylsilyl)phosphate, vinylmethyl diacetoxy silane.

In the embodiment where the acid or base formed upon hydrolysation is abase, preferably at least one of R¹, R², R³ and R⁴ is an amino group, sothat an amine is formed. More preferably one, two or three of R¹, R², R³and R⁴ are amino groups, and the remainder of R¹, R², R³ and R⁴ areother groups.

The substituents at the N atom of the amino groups preferably areselected from the group consisting of C1-C12 alkyl, C2-C12 alkenyl,C2-C12 alkynyl and C7 to C18 arenyl groups, more preferably from thegroup consisting of C1-C6 alkyl, C2-C6 alkenyl, C2-C6 alkynyl and C7 toC12 arenyl groups.

Preferred examples for the non-polymeric compound with hydrolysablesilane groups which upon hydrolysation form a base are allyl dimethyl(diisopropyl amino)silane, anilino trimethyl silane, bis(dimethylamino)vinyl methyl silane, n-butyl dimethyl (dimethylamino)silane,(diisopropylamino)trimethyl silane, octadecyl dimethyl(dimethylamino)silane and tris(dimethylamino)phenyl silane.

Preferably, the non-polymeric compound with hydrolysable silane groupswhich upon hydrolysation form an acid or a base is present in thecomposition in an amount of from 0.001 to 10 wt. %, more preferably from0.01 to 5 wt. %, and most preferably from 0.5 to 2.5 wt. %.

The composition of the second embodiment also comprises a crosslinkablepolyolefin with hydrolysable silane groups.

This crosslinkable polyolefin with hydrolysable silane groups may be acrosslinkable polyolefin with hydrolysable silane groups which uponhydrolysation form an acid or a base as described in any embodimentabove.

The crosslinkable polyolefin in the composition of the second embodimentmay also contain (only) hydrolysable silane groups which uponhydrolysation do not form an acid or a base.

Then, the hydrolysable silane compounds used for the preparation of thecrosslinkable polyolefin with hydrolysable silane groups are preferablyrepresented by the formulaCH₂═CHSi(OA)₃  (II)wherein A is a hydrocarbyl group having 1-8 carbon atoms, preferably 1-4carbon atoms.

The most preferred compounds are vinyl trimethoxy silane, vinylbismethoxyethoxy silane, vinyl triethoxy silane,gamma-(meth)acryl-oxypropyl trimethoxy silane, andgamma(meth)acryloxypropyl triethoxy silane, or combinations of two ormore thereof.

The crosslinkable polyolefin with hydrolysable silane groups may beprepared by grafting or copolymerisation in any embodiment as describedabove for the preparation of the crosslinkable polyolefin withhydrolysable silane groups which form an acid or a base uponhydrolysation.

The crosslinkable polyolefin with hydrolysable silane groups preferablycontains 0.001 to 15 wt. % of silane group containing monomers, morepreferably 0.01 to 5 wt. %, and most preferably 0.1 to 3 wt. %, and mostpreferably 0.1 to 2 wt %.

The total amount of crosslinkable polyolefin with hydrolysable silanegroups in the composition is at least 25 wt. %, more preferably at least50 wt. %.

The polyolefin compositions of the invention preferably furthercomprises a polyolefin with polar groups.

Preferably, the polar groups are selected from siloxane, amide,anhydride, carboxylic, carbonyl, hydroxyl, ester and epoxy groups.

The polar groups may for example be introduced into the polymer bygrafting of an ethylene polymer with a polar-group containing compound,i.e. by chemical modification of the polyolefin by addition of a polargroup containing compound mostly in a radical reaction. Grafting is e.g.described in U.S. Pat. Nos. 3,646,155 and 4,117,195.

It is, however, preferred that said polar groups are introduced into thepolymer by copolymerisation of olefinic, including ethylene, monomerswith comonomers bearing polar groups.

As examples of comonomers having polar groups may be mentioned thefollowing: (a) vinyl carboxylate esters, such as vinyl acetate and vinylpivalate, (b) (meth)acrylates, such as methyl(meth)acrylate,ethyl(meth)-acrylate, butyl(meth)acrylate andhydroxyethyl(meth)acrylate, (c) olefinically unsaturated carboxylicacids, such as (meth)acrylic acid, maleic acid and fumaric acid, (d)(meth)acrylic acid derivatives, such as (meth)acrylonitrile and(meth)acrylic amide, and (e) vinyl ethers, such as vinyl methyl etherand vinyl phenyl ether.

Amongst these comonomers, vinyl esters of monocarboxylic acids having 1to 4 carbon atoms, such as vinyl acetate, and (meth)acrylates ofalcohols having 1 to 4 carbon atoms, such as methyl (meth)acrylate, arepreferred. Especially preferred comonomers are butyl acrylate, ethylacrylate and methyl acrylate. Two or more such olefinically unsaturatedcompounds may be used in combination. The term “(meth)acrylic acid” isintended to embrace both acrylic acid and methacrylic acid.

Preferably, the polar group containing monomer units are selected fromthe group of acrylates.

The amount of polar group containing monomer units in the polyolefinpreferably is 40 wt. % or less, more preferably 35 wt. % or less, andstill more preferably is between 1 and 20 wt. %.

Furthermore, preferably the polar group containing monomer units arepresent in the polyolefin in an amount of from 1 to 15 mol %, morepreferably 3 to 10 mol %, and most preferably 3.5 to 6 mol %.

In a particularly preferred embodiment, the crosslinkable polyolefinwith hydrolysable silane groups in the compositions of the invention atthe same time also contains the polar groups in any of the embodimentsas described hereinbefore, i.e. the polyolefin is a terpolymercontaining both the silane groups and the polar groups.

Furthermore, also the preferred amounts for the silane group and thepolar group containing monomers as described above apply for theterpolymer.

Such terpolymers may be produced by grafting, or, preferably, bycopolymerisation of olefin monomers and unsaturated monomers containingsilane groups and polar groups.

If such a terpolymer containing both the silane groups and the polargroups is used in the composition of the invention, it is preferred thatit makes up at least 80 wt. % of the total composition, more preferablyat least 85 wt. %, and most preferably at least 90 wt. %.

In the polyolefin composition according to the invention preferably noseparately added silanol condensation catalyst different from the acidor base formed upon hydrolysation of the hydrolysable silane groups ispresent.

However, in general, such a separately added silanol condensationcatalyst different from the acid or base formed upon hydrolysation ofthe hydrolysable silane groups may be present.

If this is the case, it is preferred that separately added silanolcondensation catalyst present in an amount of 0.0001 to 6 wt. %, morepreferably of 0.001 to 2 wt. %, and most preferably 0.02 to 0.5 wt. %.

Conventional silanol condensation catalysts are for example tin-organiccompounds such as dibutyl tin dilaurate (DBTDL).

Preferably, an acidic silanol condensation catalyst is used. Such acidicsilanol condensation catalysts are disclosed for example in WO 95/17463.

The silanol condensation catalysts of the polyolefin compositionpreferably is a Brönsted acid, i.e. is a substance which acts as aproton donor.

The Brönsted acids may comprise inorganic acids such as sulphuric acidand hydrochloric acid, and organic acids such as citric acid, stearicacid, acetic acid, sulphonic acid and alkanoic acids as dodecanoic acid,or a precursor of any of the compounds mentioned.

Preferably, the Brönsted acid is a sulphonic acid, more preferably anorganic sulphonic acid.

Still more preferably, the Brönsted acid is an organic sulphonic acidcomprising 10 C-atoms or more, more preferably 12 C-atoms or more, andmost preferably 14 C-atoms or more, the sulphonic acid furthercomprising at least one aromatic group which may e.g. be a benzene,naphthalene, phenantrene or anthracene group. In the organic sulphonicacid, one, two or more sulphonic acid groups may be present, and thesulphonic acid group(s) may either be attached to a non-aromatic, orpreferably to an aromatic group, of the organic sulphonic acid.

Further preferred, the aromatic organic sulphonic acid comprises thestructural element:Ar(SO₃F)_(x)  (IV)with Ar being an aryl group which may be substituted or non-substituted,and x being at least 1.

The organic aromatic sulphonic acid silanol condensation catalyst maycomprise the structural unit according to formula (IV) one or severaltimes, e.g. two or three times. For example, two structural unitsaccording to formula (IV) may be linked to each other via a bridginggroup such as an alkylene group.

Preferably, Ar is a aryl group which is substituted with at least oneC4- to C30-hydrocarbyl group, more preferably C4- to C30-alkyl group.

Aryl group Ar preferably is a phenyl group, a naphthalene group or anaromatic group comprising three fused rings such as phenantrene andanthracene.

Preferably, in formula (IV) x is 1, 2 or 3, and more preferably x is 1or 2.

Furthermore, preferably the compound used as organic aromatic sulphonicacid silanol condensation catalyst has from 10 to 200 C-atoms, morepreferably from 14 to 100 C-atoms.

In one preferred embodiment, Ar is a hydrocarbyl substituted aryl groupand the total compound containing 14 to 28 carbon atoms, and stillfurther preferred, the Ar group is a hydrocarbyl substituted benzene ornaphthalene ring, the hydrocarbyl radical or radicals containing 8 to 20carbon atoms in the benzene case and 4 to 18 atoms in the naphthalenecase.

It is further preferred that the hydrocarbyl radical is an alkylsubstituent having 10 to 18 carbon atoms and still more preferred thatthe alkyl substituent contains 12 carbon atoms and is selected fromdodecyl and tetrapropyl. Due to commercial availability it is mostpreferred that the aryl group is a benzene substituted group with analkyl substituent containing 12 carbon atoms.

The currently most preferred compounds are dodecyl benzene sulphonicacid and tetrapropyl benzene sulphonic acid.

The silanol condensation catalyst may also be precursor of the sulphonicacid compound, including all its preferred embodiments mentioned, i.e. acompound that is converted by hydrolysis to such a compound. Such aprecursor is for example the acid anhydride of a sulphonic acidcompound, or a sulphonic acid that has been provided with a hydrolysableprotective group, as e.g. an acetyl group, which can be removed byhydrolysis.

In a second preferred embodiment, the sulphonic acid catalyst isselected from those as described in EP 1 309 631 and EP 1 309 632,namely

-   a) a compound selected from the group of-   (i) an alkylated naphthalene monosulfonic acid substituted with 1 to    4 alkyl groups wherein each alkyl group is a linear or branched    alkyl with 5 to 40 carbons with each alkyl group being the same or    different and wherein the total number of carbons in the alkyl    groups is in the range of 20 to 80 carbons;-   (ii) an arylalkyl sulfonic acid wherein the aryl is phenyl or    naphthyl and is substituted with 1 to 4 alkyl groups wherein each    alkyl group is a linear or branched alkyl with 5 to 40 carbons with    each alkyl group being the same or different and wherein the total    number of carbons in the alkyl groups is in the range of 12 to 80;-   (iii) a derivative of (i) or (ii) selected from the group consisting    of an anhydride, an ester, an acetylate, an epoxy blocked ester and    an amine salt thereof which is hydrolysable to the corresponding    alkyl naphthalene monosulfonic acid or the arylalkyl sulfonic acid;-   (iv) a metal salt of (i) or (ii) wherein the metal ion is selected    from the group consisting of copper, aluminium, tin and zinc; and-   b) a compound selected from the group of-   (i) an alkylated aryl disulfonic acid selected from the group    consisting of the structure (V):

and the structure (VI):

wherein each of R₁ and R₂ is the same or different and is a linear orbranched alkyl group with 6 to 16 carbons, y is 0 to 3, z is 0 to 3 withthe proviso that y+z is 1 to 4, n is 0 to 3, X is a divalent moietyselected from the group consisting of —C(R₃)(R₄)—, wherein each of R₃and R₄ is H or independently a linear or branched alkyl group of 1 to 4carbons and n is 1; —C(═O)—, wherein n is 1; —S—, wherein n is 1 to 3and —S(O)₂—, wherein n is 1; and

-   (ii) a derivative of (i) selected from the group consisting of the    anhydrides, esters, epoxy blocked sulfonic acid esters, acetylates,    and amine salts thereof which is a hydrolysable to the alkylated    aryl disulfonic acid,    together with all preferred embodiments of those sulphonic acids as    described in the mentioned European Patents.

The polyolefin compositions according to the invention may furthercontain various additives, such as miscible thermoplastics,antioxidants, further stabilizers e.g. water tree retardants, scorchretardants, lubricants, fillers, colouring agents and foaming agents.

The total amount of additives is generally 0.3 to 10 wt. %, preferably 1to 7 wt. %, more preferably 1 to 5 wt. %.

As antioxidant, preferably a compound, or a mixture of such compounds,is used which is neutral or acidic, must comprise a sterically hinderedphenol group or aliphatic sulphur groups. Such compounds are disclosedin EP 1 254 923 to be particularly suitable antioxidants forstabilisation of polyolefins containing hydrolysable silane groups whichare crosslinked with a silanol condensation catalyst, in particular anacidic silanol condensation catalyst. Other preferred antioxidants aredisclosed in WO2005003199A1.

Preferably, the antioxidant is present in the composition in an amountof from 0.01 to 3 wt. %, more preferably 0.05 to 2 wt. %, and mostpreferably 0.08 to 1.5 wt. %.

The non-polymeric compound with hydrolysable silane groups which uponhydrolysation form an acid or a base or silanol condensation catalyst,if present, usually is added to the silane group containing polyolefinby compounding the polymer with a so-called master batch, in which thecatalyst, and optionally further additives are contained in a polymer,e.g. polyolefin, matrix in concentrated form.

The matrix polymer is preferably a polyolefin, more preferably apolyethylene, which may be a homo- or copolymer of ethylene, e.g. lowdensity polyethylene, or polyethylene-methyl-, -ethyl, or-butyl-acrylate copolymer containing 1 to 50 wt. % of the acrylate, andmixtures thereof.

Furthermore, preferably the master batch also contains some or all ofthe additives, for example the stabilizers.

It is preferred that amount of the stabilizers contained in the masterbatch is up to 10 wt. %.

The master batch preferably is compounded with the silane groupcontaining polymer in an amount of from 1 to 10 wt. %, more preferablyfrom 2 to 8 wt. %.

Compounding may be performed by any known compounding process, includingextruding the final product with a screw extruder or a kneader.

The present invention furthermore relates to an article, such as a wireor cable, a film, or a pipe comprising the polyolefin composition in anyof the above described embodiments.

In particular, the present invention furthermore relates to a wire orcable, in particular a low, medium or high voltage cable.

A medium or high voltage cable usually comprises one or more conductorsin a cable core, an inner semiconducting layer, followed by aninsulating layer, and then an outer semiconducting layer, wherein atleast one of these layers, preferably the insulating layer, comprisesthe polyolefin composition as described above.

Insulating layers for medium or high voltage power cables generally havea thickness of at least 2 mm, typically at least 2.3 mm, and thethickness increases with increasing voltage the cable is designed for.

In addition to the semiconductive and insulating layers, further layersmay be present in medium or high voltage cables, such as a metallic tapeor wire shield, and, finally, an outermost jacketing layer.

Usually, the cable is produced by co-extrusion of the different layersonto the conducting core. Then, cross-linking is performed by moisturecuring, wherein the silane groups are hydrolysed under the influence ofwater or steam, resulting in the splitting off of alcohol and theformation of silanol groups, which are then cross-linked in acondensation reaction wherein water is split off.

Usually, moisture curing is performed in a sauna or water bath attemperatures of 70 to 100° C.

The invention relates furthermore to the use of a polyolefin compositionin any of the above described embodiments for the production of a layerof a cable.

Furthermore, the invention relates to the use of a crosslinkablepolyolefin with hydrolysable silane groups which upon hydrolysation forman acid or a base in any of the above-described embodiments, inhydrolysed form as a silanol condensation catalyst, and to the use of anon-polymeric compound with hydrolysable silane groups which uponhydrolysation form an acid or a base in any of the above describedembodiments, in hydrolysed form as a silanol condensation catalyst.

Still further, the invention relates to the use of a non-polymericcompound with hydrolysable silane groups which upon hydrolysation forman acid or a base in any of the above described embodiments, inhydrolysed form as scorch retarding agent.

The present invention is further illustrated by means of the followingexamples.

Examples

1. Measurement Methods

a) 90° C. Cross-Linking Test

In this test, the performance of an acid or base for cross-linking ofsilanol groups is tested in a standardized manner.

The cross-linkable polymer to be used in the test is a copolymer ofethylene and vinyltrimethoxysilane which has been obtained either bycopolymerisation or by grafting. The amount of vinyltrimethoxysilane inthe copolymer is 2 wt. %, the copolymer has a MFR₂ (2.16 kg/190° C.) of2 g/10 min and a density of 923 g/cm³.

A crosslinkable composition was formed by adding to this copolymer theacid or base to be tested in an amount of 4.5 mmol/kg.

This is done in the form of mixing a masterbatch consisting of LDPE and16 mmol acid/base per kg with the cross-linkable polymer so that anamount of 4.5 mmol acid/base per kg of the polymer composition isobtained.

Then, a tape is extruded from this composition. The tape with 1.7 mm inthickness is then crosslinked at a temperature of 90° C. in a waterbath.

The cross-linking degree of the composition is measured by decalinextraction according to ASTM D 2765-1, with the exception that after theextraction with decalin for 6 hours, the extraction was continuedanother 1 hour in pure, boiling decalin.

b) Melt Flow Rate

The melt flow rate (MFR) is determined according to ISO 1133 and isindicated in g/10 min. The MFR for ethylene polymers is determined at190° C. and may be determined at different loadings such as 2.16 kg(MFR₂) or 21.6 kg (MFR₂₁).

2. Example

“Free” octadecanoic acid was tested in accordance with the 90° C.Cross-Linking Test as described above.

In addition, the performance of octadecanoyloxy trimethylsilane wastested in an analogous manner as follows:

To a cross-linkable copolymer of ethylene and vinyltrimethoxysilane(amount of vinyltrimethoxysilane 2 wt. %, MFR₂ (2.16 kg/190° C.) 2 g/10min, density 923 g/cm³) octadecanoyloxy trimethylsilane was added.

The silane compound was added in form of a masterbatch of LDPEconsisting of 16 mmol silane compound per kg masterbatch.

The masterbatch was added in such quantity that the amount of silanecompound in the final composition was 4.5 mmol per kg of polymer. A tapewas extruded (1.7 mm in thickness) and was then crosslinked at atemperature of 90° C. in a waterbath.

The crosslinking degree of the composition was measured by decalinextraction according to ASTM D 2765-1, except that, after decalinextraction for 6 h, extraction is continued for 1 h in pure, boilingdecalin.

The crosslinking degree obtained with “free” octadecanoic acid and withoctadecanoyloxy trimethylsilane is shown in Table 1.

TABLE 1 Crosslinking degree (%) 90° C. Compound 1.5 h  5 h 20 h 40 hOctadecanoyloxy 0 11 60 67 trimethyl silane Octadecanoic acid 1 30 64 73

This result shows that the acylsilane acts as a water scavenger andproduces octadecanoic acid when reacting with water, which then works asa silanol condensation catalyst. The final crosslinking degree is thesame as with octadecanoic acid. The delay in reaction also shows thescorch retarding effect of using octadecanoyloxy trimethyl silane issubstantial.

The invention claimed is:
 1. A polyolefin composition comprising (i) acrosslinkable polyolefin with hydrolysable silane groups, thecrosslinkable polyolefin being prepared from a hydrolysable silanecompound represented by Formula (II):CH₂═CHSi(OA)₃  (II) wherein A is a hydrocarbyl group having 1-8 carbonatoms; and (ii) a non-polymeric compound with hydrolysable silane groupswhich upon hydrolysation form an acid or a base, wherein thenon-polymeric compound has 50 repeating units or less, and isrepresented by Formula (III):R¹SiR²R³R⁴  (III) wherein R¹ is a non-hydrolysable hydrocarbyl groupwhich comprises 2 to 30 carbon atoms, which further comprises at leastone unsaturated group, and optionally contains heteroatoms; and R², R³,and R⁴ are organic residues which may be the same or different from eachother with the proviso that at least one thereof is an acyloxy groupthat will generate a carboxylic acid upon hydrolysation or is an aminogroup that will generate an amine upon hydrolysation; characterised inthat the carboxylic acid or amine is one which generates a gel contentof at least 40% after 40 hours in a 90 ° C. cross-linking test which isconducted as follows: a copolymer of ethylene and vinyltrimethoxysilanehaving a MFR₂ (2.16 kg, 190° C.) of 2 g/10 min, a density of 923 g/cm³,and 2 wt.% of vinyltrimethoxysilane is provided, to which the carboxylicacid or amine is added in an amount of 4.5 mmol/kg to form acrosslinkable composition, said crosslinkable composition is formed to atape of 1.7 mm thickness, and said tape is cross-linked in a waterbathat a temperature of 90 ° C.
 2. An article comprising the polyolefincomposition according to claim
 1. 3. A method for forming a silanolcondensation catalyst by hydrolysing a non-polymeric compound withhydrolysable silane groups in a polyolefin composition, wherein thehydrolysable silane groups form an acid or a base upon hydrolysationwhich acts in hydrolysed form as a silanol condensation catalyst andwherein the polyolefin composition comprises: (A) a crosslinkablepolyolefin with hydrolysable silane groups, the crosslinkable polyolefinbeing prepared from a hydrolysable silane compound represented byFormula (II):CH₂═CHSi(OA)₃  (II) wherein A is a hydrocarbyl group having 1-8 carbon;and (B) a non-polymeric compound with hydrolysable silane groups,wherein the non-polymeric compound has 50 repeating units or less, andis represented by Formula (III):R¹SiR²R³R⁴  (III) wherein R¹ is a non-hydrolysable hydrocarbyl groupwhich comprises 2 to 30 carbon atoms, which further comprises at leastone unsaturated group, and which may contain heteroatoms; and R², R³,and R⁴ are organic residues which may be the same or different from eachother with the proviso that at least one thereof is an acyloxy groupthat will generate a carboxylic acid upon hydrolysation or is an aminogroup that will generate an amine upon hydrolysation.
 4. Polyolefincomposition according to claim 1 which further comprises a polyolefinwith polar groups.
 5. Polyolefin composition according to claim 1wherein the acid or base formed upon hydrolysation is selected from thegroup of carboxylic acids, sulphonic acids, phosphorous acids, halogenacids, oxoacids, oximes, imines and amines.
 6. Polyolefin compositionaccording to claim 1 wherein the crosslinkable polyolefin withhydrolysable silane groups comprises a polyethylene with hydrolysablesilane groups.
 7. Polyolefin composition according to claim 1 wherein inthe crosslinkable polyolefin with hydrolysable silane groups are presentin a total amount of 0.001 to 15 wt. %.
 8. Polyolefin compositionaccording to claim 1 wherein the crosslinkable polyolefin withhydrolysable silane groups also contains polar groups.